Surface coating UV curing technology explained (4)

9. Basic research on UV curing technology

The final part of this article describes typical examples of basic research in UV curing currently conducted in Australia. The reason why such research needs to be carried out is not only because of the need for new chemistry for the promotion and application of UV curing from an industrial point of view, but also to solve the problems that have emerged in applications such as banknotes. Some of the problems found are listed in Table 8. While the current current banknotes are satisfactory, improvements such as improving the adhesion between the primer and the polypropylene and improving the adhesion of the matte finish to the primer are still needed. In addition, the presence of a matting agent in the topcoat has had an adverse effect on the useful life of the note. These comments highlight a major problem that commonly occurs when UV printing and coating on plastics such as polypropylene. In the printing work, in order to improve the adhesion of the UV ink to the polypropylene, a thin UV curing primer is applied before the ink is applied. If the coating can be grafted onto the substrate at the same time as the coating is cured, such problems occurring when the ink and coating are cured on the substrate, especially on the plastic substrate, can be solved. Curing is the polymerization of oligomers and monomer mixtures on the substrate. The original bond between the coating and the substrate surface is essentially a weak physical force, while the grafting involves chemical bonding. Carbon-carbon bonds are formed. Therefore, it is important to have a condition where grafting can occur during curing. There is not much basic work that can be applied to this purpose. In Australia, the purpose of the current research work of the authors of this article is to solve this problem.
Table 8 - Additional aspects of current polymer banknotes need to be improved
1. Improve the adhesion between polyolefin and UV luster primer.
2. Improve the adhesion between UV primer and UV matte finish.
3. Increase the wear resistance of UV matte finish. 
Some of the results obtained from the above studies on the basis of the data are shown in Table 9. The grafting rate in the table is the data of grafting of methyl methacrylate methanol solution onto polypropylene and cellulose under UV conditions. Methyl methacrylate is often used as a typical monomer because it represents methacrylic esters and it has been the most commonly used UV chemical in the industry. Solvents such as methanol are commonly used to disperse monomers and can also make the difference in grafting rates that may result from different photoinitiators more pronounced. In order to make the difference more prominent, a low-power 90wUV lamp was used in order to slow down the polymerization. The results in Table 9 show the arrangement of the grafting rates with various photoinitiators. Irgacure 184 is the photoinitiator that makes grafting the most active, but the actual formation is homopolymer (determined by the change in viscosity of the monomer solution), and Irgacure 1800 can result in a maximum graft rate.
Table 9 - Grafting rate of grafting ratio of cellulose and polypropylene to MMA (60% methanol solution) (%) (a)
Photoinitiator cellulose polypropylene 8
Comparison sample 6.0e 1.4
1800b 59f 13.7 
1700b 70g 9.8 
184b 59f 17.6 
BEEc 49g 14.7 
AIBN 41g 3.6
a. The drop in homopolymer yield, 1800 is the highest;
b. Ciba Geigy Irgacure products;
c.BEE = benzoin ethyl ether;
d.AIBN=azobisisobutyronitrile;
e. Irradiation 16hrs to 90W, 24cm (lamp length) high pressure UV light;
f. Irradiation 5hrs;
g. Irradiation 3hrs

These results are consistent with the UV curing studies performed by the authors. The monomer/oligomer mixtures used here include epoxy acrylates, urethane acrylates, and polyester acrylates and are used as dilution sheets. The body of TPGDA (tripropylene glycol diacrylate); the mixture is cured on the substrate, and the efficiency of the photoinitiator during curing is judged by the yield of the homopolymer. The results obtained by the basic research institute and the assumption model initiated by the authors of this paper are consistent with the distribution of the polymerization mixture between the grafting solution and the substrate. The result of industrial application shows that grafting and curing can be accomplished simultaneously; the photoinitiator is important on the one hand, because the activity of the radical generated by the photoinitiator is the key to rely on this activity to capture hydrogen on the surface of the substrate. Atoms lead to grafting.

10. Conclusion
UV curing technology has developed into a technology based on industry, its growth rate is 10-12% / year. It is the main end user of this technology in Australian coatings and inks, which is consistent with the development of other countries in the world. The application of this technology in the packaging industry and etching industry is also particularly popular. It should be noted that UV curing has potential for application to any substrate and can be applied to many other suitable markets. Although it uses less, it still has a strong hope of generalization. UV-curable polymer banknotes are a unique development in China. They are very successful. The Reserve Bank of Australia, which developed the banknotes, can also use it to export foreign exchange.